Polkadot JAM Slides - Token2049 - By Dr. Gavin Wood
Grow Your Own, Nevada! Summer 2013: What is Wrong with My Plant?
1. What’s wrong
with my plant?
How to Diagnose
Plant problems
Wendy Hanson Mazet
University of Nevada
Cooperative Extension
hansonw@unce.unr.edu
2. First - What is the problem?
Is there a pest present?
Anything or anyone that is
detrimental to your
garden is a pest
destroys crops &
structures
poses health threats to
family or pets
reduces aesthetic value
of your property
3. Anticipates and prevents
damage
Uses several tactics in
combination
Improves effectiveness,
reduces side effects
Relies on identification,
measurement, assessment,
and knowledge
Integrated Pest Management
IPM: a balanced, tactical approach
5. How to have success
Six Key Steps
Observation
Early Detection
Correct Identification
of insect or pest or issue
Education / research
Select appropriate
control or action
Proper application
6. What is causing the problem?
Biotic
Living Agents
Viruses
Fungi
Bacteria
Abiotic
Non-Living Agents
Soil issues
Nutrient Deficiencies
Temperature
Wind
Water
Sun
Shade
7. Biotic
Signs of Fungal Diseases
Visible organism
Rust pustules
Mycelial groth (fuzzy
spots)
Spores
Black fungal bodies
Bacterial diseases enter
through wounds
Symptoms
lesions,
Wilt
Discoloration of leaves
Galls
Slimy wet areas or roots
8. Biotic cont.
Virus
Virus infects plant tissue
and cells
Symptoms:
Mottling
Mosaic color patterns
Purple coloring
Stunting
Distortion
Insects, mites, fungi or
nematodes often are
the carrier of viruses
and transmit the virus.
9. Powdery Mildew
Fungal Disease
Looks like talcum
powder or flower dust
on leaf tissue
Use of resistant varieties
Avoid poor air
circulation
Over abundant shade
Avoid excessive
overhead watering and
night time foliar watering
Change the
environment first, before
turning to fungicides
10. Determine causes of plant damage
Environmental injury
includes:
Temperature extremes:
too hot, too cold
Lightning
Storm situations with hail,
wind or tornado
Wind injury
Frost damage
Moisture extremes: too
much, too little
Monitor the area since
some environmental
injuries can resemble
other biotic or abiotic
causes.
Abiotic causes of plant
damage are non-living.
Generally are distributed
uniformly across a plant
or field and are
repeated
Don’t spread or move
with time
May be from
mechanical factors such
as cultivator injury;
physical factors such as
environment; or
chemical factors such as
pesticide or nutrient
problems
11. Cucumber
Scenario:
2 p.m.
96 degrees Fahrenheit
Watered at 7 a.m.
insects?
disease?
environmental?
Where do you start?
12. Biotic vs. Insects
Identifying Insects
Symptoms may
include tunnels in
stalks or leaves, holes
in leaves, chewed
leaves, galls, leaf
curling, speckling or
separation of leaf
tissue.
Biotic
Look for pattern
Mottling, mosaic or
stippling or leathery
leaves
Do not over head
water in the evening
hours
Take photos and bring
in a sample for lab
testing.
13.
14. Control Tactics of
insect damage
Five Most Common
Mechanical
Cultural
Physical
Genetic
Chemical
15. Insects - Know Your Enemy:
Sucking Insects: Pierce and suck plant juices
Yellow or bronze discoloration of leaves and shoots
Wilting and curling of leaves and shoots
Aphids, whiteflies, mites (not true insects) feed near tip of
young shoots and on undersides of leaves
Leafhoppers feed under leaf surfaces, and scale feed on
leaves, stems, and shoots.
All feed in large groups except leafhoppers
16.
17. Piercing Insects
Control:
Keep plants healthy
Maintain a diverse habitat
Monitor garden daily, so
when insect pests are
found, control measures
can be taken quickly.
hose plants off
insecticidal soap sprays
horticulture oils including
Neem oil
Cover with garden blanket
Encourage Beneficial insects
18. Aphid
• Small, soft bodied
insects 1/10 inch long
• Long mouth parts
used to suck plant
juices.
• Cornicles are found
on most species.
• Found in many colors.
• Most over-winter as
eggs, hatch in spring.
• Secrete honeydew.
• Most abundant in
cool spring and cool
fall.
• Ants may be present
tending aphids.
19. NCCE
Stink Bugs
•5/8 inches long,
bright green,
brown with stripes,
large body small
head
• When crush they
stink!
•Piercing/sucking
mouth parts
• stippling damage
on leaf and stem
tissue
•Barrel like eggs laid
on leaf and stem
tissue
20.
21. University of Minnesota Extension
Squash
Bugs
•5/8 inches long, brown
with stripes, large
oblong body small
head
•Overwinter as adults
•Piercing/sucking mouth
parts
• stippling damage on
leaf and stem tissue
•Eggs are rust to root-
beer colored and
found on the
undersides of the leaf
22. Spider Mites
•Very small
•Not an insect.
• Arachnid possessing 8
legs.
•When spider mites feed
on fruit can cause a
silvery or bronzy sheen
called russetting.
•When populations are
large a fine webbing
may be seen on leaves
and needles.
•Prefer hot, dry and dusty
environments.
23. Mites Cont.
Several species of mites in our
area.
Common is two-spotted
spider mite
Found on outdoor plants, garden
areas and houseplants.
Spruce spider mite
Found on Juniper, Spruce, and
other needled-leaf evergreens.
Clover mite-pest of lawns and
weedy areas
Occurs as periodic lawn pest,
and nuisance in spring and
Fall.
24. Adults a wedge-shaped
and about 1/8 inch long
Leafhoppers over-winter
as eggs on twigs, or as
adults in protected sites,
such as bark crevices.
Very active – jumping,
flying and running when
disturbed.
Sucking mouthparts
cause stippling, yellow
to brown leaves.
Nymphs are considered
more damaging than
adults
Leafhoppers
25. Slender insects with
fringed wings as
adults
Less than 1/20 inch
long
Feed within buds
and furled leaves or
in other enclosed
parts of the plant
Thrips are poor fliers
but can readily
spread long
distances by floating
with the wind
Thrips Orius
insidiosus
26.
27. White Moths with
powdery wings 1/10th
inch in length
• Eggs are typically laid
on the newest leaves.
• Piercing-sucking
method of feeding
produces stippling of
leaves
• Heavy feeding may
wilt and stunt plants
• Hosts, bean,
cucumber, eggplant,
lettuce, okra, potato,
tomato, squash, and
sweet potato.
Whitefly
28.
29. Chewing Insects
Loopers, hornworms,
leafrollers, cutworms
are all larvae of
butterflies and
moths.
Control:
Handpick larvae
Cover with garden
blanket
Bacillus thuringiensis-Bt
Encourage Beneficial
insects
Cabbageworm Butterfly
30. Earworm
• ½ inch to1 inch in length
• Prefer cool damp places
hiding in organic mulch,
under bark, in garden
debris
• Feed on a variety of
dead and living
organisms, including
insects, mites and shoots
of plants.
• Earwigs also feed on silks
of corn, causing poor
kernel development.
31. Miller Moth
Army
Cutworm
•Smooth, gray-black
with smooth skin
•Can reach 1 ½ inches
long
•When disturbed they
curl into a C shape
•Caterpillars chew on
stems and leaves
•Adult is known “Miller
Moth”
32.
33. • 1 ¾ inches long
• Light green to
brownish black
• Alternating light
stripe running down
the length of its body
• Chews holes in
leaves and fruit
• Eggs laid on the silk
• Adult is a moth and
overwinters as a
pupa
Corn
Earworm
34. Cabbage &
Alfalfa looper
•Loopers 1 to 1.5”
•feed on leaves
•Female can lay 200-350
eggs over a 12 day
period – hatching occurs
within 2 weeks
•Larvae will feed 2 to 4
weeks
•Note: plants can lose 20
to 25 percent of their leaf
area without a reduction
in yield
35.
36. •Large green to
brown caterpillars –
up to 3-4 inches
•Can defoliate a
tomato within days
•Adult is a sphinx
moth – known as a
hummingbird moth
•Pupa overwinter in
the soil
Hornworm
37.
38. Leafminers
•Larvae a maggot or slug
like and burrow between
the two layers of tissue
•Adult small slender-bodied,
grayish, black-haired fly –
¼” long
•Larvae will pupate in the
top 3” of soil, but some will
stay in the leaf itself
•3 to 4 generations a year
•Larvae over winter in the
soil
39.
40. The Good Guys –
Naturally occurring predators
Lady beetles Snake fly
Green lacewing
42. Predators available for purchase
Convergent lady beetle
Spined soldier bug
Praying mantid eggs
Green Lacewing Eggs
43.
44. Cracking
Tomatoes often start to
crack during warm, rainy
periods particularly
following a lengthy dry
spell.
Fruits crack by expanding
too fast, usually when they
are fully-size and beginning
to color.
To avoid cracking:
select resistant varieties
keep soil moisture as even
as possible
avoid excessive dryness or
wetness through-out the
growing season
45. Blossom End Rot
Physiological disorder that
occurs on tomato, pepper,
eggplant & summer squash.
On zucchini and other summer
squash, the blossom end of the
fruit begins to rot and within a
short time the entire fruit has
rotted.
Caused by a lack of calcium in
the developing fruit. The
deficiency typically occurs
during high summer
temperatures
No need to apply calcium to the
soil.
Try to maintain an even moisture
supply by watering
Uneven moisture supplies and
excessive nitrogen can inhibit
calcium uptake.
46. Catfacing
Characterized as unusual,
and sometimes bizarre,
swellings, streaks of scar
tissue, caused by
abnormal development of
the tomato flower at
blossom time.
Cool weather is believed
to cause the flower
problems.
47. Summer Squash
Fruit Abortion
Improper
pollination of
the flower
Encourage
beneficial
insects
Plant umbel
flowers near
squash plants
48. Sunburn/sunscald
Hot afternoon sun
When green or ripening
tomatoes get too much sun
exposure
The first symptom is a
yellowish-white patch that
appears on the side of the
fruit facing the sun.
The area will enlarge as the
fruit ripens and become
grayish-white.
Maintain adequate foliage
on the plants
Add additional shade if
possible
Grow plants in tomato cages,
49. Voles
•Also called meadow,
field or pine mice
•4 to 8.5 inches long
•vary in color from
brown to gray
•Large colonies
•Damage by voles
can be reduced by :
•habitat
modification
•exclusion
•Repellents
•Trapping
•poison grain baits
•http://www.ipm.ucdavis.edu/PMG
/PESTNOTES/pn7439.html
UC IPMMissouri Botanical
Garden
Curtis, B, D.
Curtis, and W.
Miller. 2009
50. http://ucanr.org/sites/ipm//ipmweb/?p
=/PMG/PESTNOTES/pn74161.html
House & Deer
Mouse
•House Mouse
•vary in color gray,
light brown to black
•Short hair, with small
eyes and large ears
•Life span 9 to 12
months
•Deer Mouse
•Two-tone, brown to
grey on top with a
white belly. Tail 50/50
tan and white
•Start reproducing at
6 weeks of age
•Prefer seeds, but will
eat fruits,
invertebrates and
fungi
51. Woodrats
Important Facts:
•Also known as pack
rats, bushy tailed
wood re and trade
rat
•Active at night
•Build stick dens on
the ground or in
trees
•Herbivores, green
vegetation, twigs
and shoots
•1 litter per year –
litter size ~ 4
52. Rats
Nocturnal
Requires water daily
Will travel several hundred
feet from nest
Prefer to travel on edges
Wary of new objects in the
environment
They can jump, swim and
squeeze into and through
almost anything
Will eat pipes, wire, blocks,
and whatever necessary
to get to food
Mice
Nocturnal
Generally get water from
food source
Will travel long distances
from nest
Prefer to travel on edges
Not wary of new objects in
the environment
They can jump, swim and
squeeze into and through
almost anything
Live outside, in homes a
sheds
53. Moles
Important Facts:
•live in underground
runways
•Seldom seen above
the ground
•Runways 5 to 20”
deep
•Prefer loose, moist soil
•4 to 8” long - Blind
•Carnivores –
earthworms, grubs,
beetles, insect larvae
•Can eat 40lbs of food
a day
•Single litter – 3 to 5
•Life span – 3 years
54. Quail & Birds
Important Facts:
•Generally prefer
open space during
breeding
•Omnivorous, but tent
to be vegetarians
looking for seed and
seedlings
•1 clutch per year –
clutch size ~12 eggs
•When quail reach 2
months old they can
breed
•Life span 3 – 5 years
55. Resources
http://icwdm.org/handbook/index.asp
www.ipm.ucdavis.edu
Nevada Department of Wildlife
http://www.ndow.org/
775-688-1500 Reno office
For general questions or comments ndowinfo@ndow.org
Nevada Department of Agriculture
http://agri.state.nv.us/
405 South 21st Street, Sparks, NV 89431
775-353-3638
56. Companion
PlantingPlanting a variety of flowers
will attract many of the
beneficial insects.
•Flowers in the
sunflower (Asteraceae)
family consist of many
small flowers which
attract many beneficial
insects.
•Carrot family
(Apiacea)
•Buckwheat family
(Polygonaceae)
•Scabiosa family
(Dipsaceae)
57. Simple rules for crop rotation:
• Don’t follow tomato, peppers or eggplant with
potatoes, or each other.
• Allow 3 years before replanting the same group
in any given bed.
• Onions may be planted throughout all groups.
• Beets, carrots and radishes may be planted
among any group, and replanted as early
crops are removed.
• Interplant with companion plants to minimize
pesticide use.
• Keep good records so you can duplicate
successes.
PennState – Crop Rotation
58.
59. Incorporating age old techniques
Biochemical Pest Suppression
Some plants exude chemicals from roots or aerial parts
that suppress or repel pests and protect neighbouring
plants.
Insect Plants that should deter
Aphid Chives, Coriander, Nasturtium
Ants Tansy
Asparagus
Beetle
Pot Marigold
Bean Beetle Marigold, Nasturtium, Rosemary
Cabbage Moth Hyssop, Mint (also clothes moths),
Oregano, Rosemary, Sage,
Southernwood, Tansy, Thyme
60. Insect Plants that should deter
Potato Bugs Horseradish
Mosquitoes Basil, Rosemary
Moths Santolina
Squash Bugs &
Beetles
Nasturtium, Tansy
Tomato Horn
Worm
Borage, Pot Marigold
Carrot Fly Rosemary, Sage
Flea Beetle - Catmint, Mint
Flies - Basil, Rue
Japanese
beetles
- Garlic & Rue (When used near
roses and raspberries), Tansy
61. Season extender-shade-rodent
and insect exclusion
Row Cover
Burlap
Wall-of-waters
Sheets, or blankets
Don’t forget to save old
tomato cages
Commercial cold
frames
Or homemade cold
frames of Plexiglas,
old windows, or sheet
plastic
Management Of Thrips In Greenhouse Crops Agdex#: 290/621 Publication Date: 09/03 Order#: 03-075 Last Reviewed: 08/09 History: Replaces OMAFRA Factsheet Pest Management of Thrips in Greenhouse Vegetables, Order No. 94-023 Written by: Graeme Murphy - Greenhouse Floriculture IPM Specialist/OMAFRA; Gillian Ferguson - Greenhouse Vegetable IPM Specialist/OMAFRA; and Les Shipp - Greenhouse Entomologist/ Agriculture and Agri-Food Canada Table of ContentsIntroductionManagement StrategiesMonitoringCultural ControlPhysical ControlBiological ControlChemical ControlRelated LinksIntroductionThrips are a major pest of greenhouse crops in Ontario. A number of thrips species are commonly found including western flower thrips (Frankliniellaoccidentalis), eastern flower thrips (Frankliniellatritici), onion thrips (Thripstabaci), and Echinothrips. However, the western flower thrips is the predominant species and the most difficult to control. See OMAFRA Factsheet Order No. 03-077, Biology of Thrips in Greenhouse Crops for a detailed description of the pest. Management StrategiesMonitoringMonitoring the population levels of western flower thrips is critical for successful pest management. In vegetable crops, begin monitoring during propagation and continue after transplanting. In floriculture crops, thrips can be present at damaging levels year-round, although populations are usually reduced during winter. Use commercially available blue or yellow sticky traps to monitor the population densities of adult thrips (Figure 1). Blue traps are more attractive to western flower thrips, although yellow traps are more attractive to other pests such as whiteflies and aphids. The choice depends on how many pests need to be monitored, the susceptibility of the crop to thrips and/or tospoviruses, and the need to detect thrips populations at low levels. Figure 1A. Blue sticky card.Figure 1B. Yellow sticky card.When setting up a monitoring program use 1 trap per 100-200 m2. The exact number will depend on the layout of the greenhouse. A large open range will require a lower total density of cards than a greenhouse made up of a number of smaller areas. Place the sticky cards in a grid pattern throughout the greenhouse. Check the traps weekly to record the average number of thrips per trap per week. Be aware that this is not an absolute measure of the population - it measures population trends, the increases and decreases in thrips numbers throughout the year. As you become more aware of how the numbers on sticky cards relate to the population in the crop, you can use the monitoring data to assist in making pest management decisions. There are precision-level sampling programs for monitoring adult western flower thrips on sweet pepper and cucumber. These sampling programs vary the number of samples taken according to the population level of the pest, and accurately predict the pest density to set precision levels. Contact an OMAFRA Greenhouse Pest Management Specialist or your IPM consultant for more detailed information before implementing your monitoring program. Cultural ControlSanitation is the first and most important step in implementing an effective pest management program. Effective sanitation reduces or even eliminates thrips as a pest problem. For example, in cut roses, removing all flower buds (including non-marketable flowers) can significantly reduce thrips populations in that crop. For more detailed information on implementing an effective sanitation program in greenhouse vegetables, see OMAFRA Factsheet, Order No. 94-029, Sanitation Recommendations For Management of Insect & Mite Pests of Greenhouse Vegetables. Cultural control measures also include maintaining a healthy crop and an optimal greenhouse environment (such as 80% RH) that would provide less favourable conditions for a rapid increase in population densities of thrips.Physical ControlInsect exclusion screening restricts the movement into the greenhouse of many common greenhouse crop pests including thrips, removing an important variable from a grower's pest management program. The influx of pests from outside can overwhelm an IPM program, making it difficult for a grower to plan ahead. For more information on screening, see the OMAFRA Factsheet Order No. 00-021, Screening of Greenhouses for Insect Exclusion. Biological ControlBiological control of thrips is used more frequently and more successfully in greenhouse vegetables than in floriculture production. However, an increasing number of flower growers are also using this strategy with success. Predatory mites (Neoseiulus (=Amblyseius) cucumeris, Iphesius (=Amblyseius) degenerans and Hypoaspis spp.) and minute pirate bugs (Oriusinsidiosus) provide effective biological control of thrips. N. cucumeris is the most extensively used of the predatory mites (Figure 2). N. cucumeris controls western flower thrips by feeding only on the first instar larvae. As such, it takes a number of weeks for the impact of this predator to be seen in the greenhouse, and it is unlikely that it will completely eliminate thrips populations. The life cycle for N. cucumeris is completed in approximately 10 days at 20°C and 6 days at 25°C. Figure 2. Adult and egg of Neoseiuluscucumeris.Introduce predatory mites at the beginning of the crop or as soon as thrips are detected. Sanitation at the end and beginning of a cropping season is extremely important and will delay any thrips infestation until the biological control agents can be effective. Regular introductions of N. cucumeris are necessary, either by dispersing bran on plants or growing medium, or by hanging a bran bag rearing system on plants (Figure 3). The bag system provides a continuous release of mites to the plant and should be replaced monthly. The number of introductions of N. cucumeris depends on the crop and level of thrips infestation (contact an OMAFRA Greenhouse Pest Management Specialist or your IPM consultant). Control of the thrips should be achieved in 5-9 weeks. When using N. cucumeris, it is important to maintain at least 70% RH in the greenhouse and not to have used any persistent pesticides such as carbamates or synthetic pyrethroids for several months. Orius is effective in controlling thrips (Figure 4). Unlike N. cucumeris, Orius feeds on all stages and is often found in the flowers where it feeds on pollen as an alternative food source. Orius does not seem to be as effective in flower crops as it is in vegetables. Development time from egg to adult is 31 days at 20°C and 19 days at 25°C. Orius enters reproductive diapause when daylength is less than 12 hours per day. Thus, Orius is only effective as a biological control agent from March to September.Figure 3A. Introduction of predatory mites directly onto the plant.Figure 3B. Introduction of predatory mites using the bag rearing system.Figure 3C. Introduction of predatory mites onto the growing medium from where they will move onto the foliage of young plants.Figure 4. Adult Oriuspreying on western flower thrips.The introduction rate for cucumber and sweet pepper is 0.5-1 Orius per plant when the pest level is low. One or two releases are usually sufficient to provide thrips control in approximately 3-5 weeks. Orius are introduced as adults in several locations and allowed to naturally disperse by flying throughout the greenhouse. Flower sampling is the best method to monitor the presence of Orius. Orius at 2.5 per cucumber plant also provides effective control in 3-6 weeks when the population levels of thrips are high (5-9 per flower). Iphesiusdegenerans (Figure 5) differs from N. cucumeris in appearance and in being able to tolerate less humid conditions. It is dark, very agile and reproduces very well on pollen. Therefore it performs best in crops with a pollen source, e.g. greenhouse peppers, and is unlikely to be the best option for floricultural crops. It can be reared in the greenhouse on castor bean plants (which produce large amounts of pollen) that may be used as release points for the predator within the greenhouse. Figure 5.Iphesiusdegenerans.Hypoaspis is a soil-dwelling predatory mite that feeds on a variety of soil organisms, including thrips pupae (Figure 6). Apply as a once-only application to the growing medium (e.g., rockwool, peat mixes) at the beginning of the crop. It is difficult to determine the exact impact of Hypoaspis on a thrips population, but it is better used in combination with other predators and is unlikely to provide sufficient control on its own. Figure 6. The predatory mite, Hypoaspis. Chemical ControlChemical control of western flower thrips can be difficult. They are resistant to most pesticides and feed deep within the flower head or on developing leaves. This makes them a difficult target for insecticides, so thorough coverage is essential. General recommendations regarding pesticide use for thrips control are as follows: At the action threshold (when thrips population levels dictate spraying to prevent economically damaging numbers from appearing), spray 4-5 days apart for 3 consecutive applications.Rotate chemical classes and use a single chemical class only for the duration of the thrips' life cycle. This generally means using a different class every 2-3 weeks depending on time of year. Generation times are longer at cooler temperatures.Apply pesticides in early morning or late afternoon, when flight activity of thrips is at a peak. This increases exposure of the thrips to the pesticides.
We’re all familiar with earwigs, but controlling these shy, nighttime insects can be difficult. They feed on a wide variety of living plant material, including vegetable fruits and foliage. Recently earwigs have been observed in the heads of leafy greens, which would warrant control. They can also feed on the soft flesh of developing sweet corn.Keep in mind that earwigs are also beneficial predators of mites and soft-bodied insects and insect eggs, so they are not all bad.In the home garden, trapping earwigs is an alternative to insecticides. Use tuna cans filled with 1/2-inch of fish or vegetable oil or bacon grease. Dump out trapped insects and refill can regularly. Rolled up newspaper or corrugated cardboard will also attract insects for hiding during the day. Empty into a can of soapy water regularly.Treatment: If control close to harvest is warranted, products with insecticidal soap or pyrethrin have a PHI of 12-24 hours.
Management (Back to Top)Sampling. Several methods for population assessment have been studied, and collecting puparia in trays placed beneath plants was recommended by Johnson et al. (1980b) as a labor-saving technique. Zehnder and Trumble (1984) used yellow sticky traps to monitor adults, and reported that Liriomyzasativae flies were more active at the middle plant height of tomatoes, while Liriomyzatrifolii was more active at low plant height. They also confirmed the value of pupal counts for prediction of adult numbers two weeks later. Yellow sticky traps, however, have the advantage of being able to quickly detect invasion of a field by adults from surrounding areas. Sequential sampling plans were developed by Zehnder and Trumble (1985). Insecticides. Foliar application of insecticides is often frequent in susceptible crops Insecticide susceptibility varies greatly both spatially and temporally. Many organophosphate and carbamate insecticides are no longer effective. Insecticides are disruptive to naturally occurring biological control agents, and leafminer outbreaks are sometimes reported to follow chemical insecticide treatment for other insects. Insect Management Guide for vegetablesCultural practices. Some crops vary in susceptibility to leaf mining. This has been noted, for example, in cultivars of tomato, cucumber, cantaloupe, and beans (Hanna et al. 1987). However, the differences tend to be moderate, and not adequate for reliable protection. Nitrogen level and reflective mulches are sometimes said to influence leafminer populations, but responses have not been consistent (Chalfant et al. 1977, Hanna et al. 1987). Placement of row covers over cantaloupe has been reported to prevent damage by leafminer (Orozco-Santos et al. 1995). The same study evaluated the benefits of transparent polyethylene mulch, and found no reduction in leafminer populations. Sometimes crops are invaded when adjacent crops are especially suitable, as was reported by Sharma et al. (1980) in California, where cotton was an important source of invaders. Weeds are a source of flies (Parkman et al. 1989), but also a source of parasitoids.