B7 - Caterpillars

Introduction

Caterpillars are the larvae of moths and butterflies.  Various species can damage both protected and outdoor herbs.  The presence of caterpillars and the damage caused by their feeding can make fresh cut and potted herbs unmarketable.

  

Recognition and host plants

Caterpillars can be from a few mm up to 4 cm long, depending on species and age.  They have a head capsule, three pairs of small ‘true' legs at the front end and, usually, five pairs of larger, fatter ‘prolegs' further back, one pair being at the rear end.  Some species have a reduced number of prolegs. There is a row of spiracles (breathing holes) down each side of the body. 

The main caterpillars infesting protected herbs are:

1.  Three species of ‘cabbage caterpillars', which attack cruciferous herbs e.g. rocket, and also cruciferous weeds e.g. charlock and shepherd's purse.

• Large white butterfly (Pieris brassicae) which is hairy and yellow with black spots (Fig.1). These caterpillars often occur in groups.
• Small white butterfly (Pieris rapae) which is velvety green with a narrow yellow stripe along the back (Fig. 2).
• Cabbage moth (Mamestra brassicae) which is fat, smooth and mottled green, grey or brown, with a paler underside (Fig. 3).

2. Silver Y (Autographa gamma) caterpillars can damage various herbs including basil, parsley and brassicas.  They are green with dark stripes along the back and a narrow yellow line down the side (Fig. 4). These caterpillars have only three pairs of prolegs, instead of the usual five pairs.

3.  Angle-shades moth caterpillars (Phlogophora meticulosa) can attack protected herbs e.g. basil and parsley and are green or brown with V-shaped marks on the back (Fig. 5).

4. Mint can be attacked by the ‘mint moth', Pyrausta aurata, small, leaf-rolling, green or purple caterpillars with yellow stripes and black spots (Fig. 6).  The adult moths have brownish-purple wings with golden spots (Fig. 7).  They fly by day and can be seen sitting on plants or flitting between them.  

5. Carnation tortrix moth (Cacoecimorpha pronubana) is another small leaf-rolling caterpillar which can damage some herbs including mint.  The caterpillar is small, yellowish-green with a pale brown head capsule (Fig. 8), and wriggles backwards when touched.  

6.  Soil-grown crops could be damaged by cutworms, e.g. the caterpillar of the turnip moth, Agrotis segetum.  This is a plump, greyish-brown caterpillar with faint darker stripes (Fig. 9), found in the soil around plants that have been severed by its feeding damage.  However, cutworm attacks are uncommon on protected herbs.        

Symptoms

Many caterpillars chew holes in the leaves (Fig. 10).  Young caterpillars e.g. angle-shades moth can produce 'windows' in the leaf, where only the underside is removed, leaving the upper surface intact.  Caterpillar damage can usually be distinguished from slug or snail damage by the presence of caterpillar droppings (‘frass', often barrel-shaped) on the leaves (Fig. 11) and the lack of slime trails.  The leaf-rolling and webbing caterpillars (carnation tortrix moth and mint moth) roll the leaves up and spin them together with silk to form a shelter in which to hide and feed (Figs 8 and 12).  Cutworms such as the caterpillars of the turnip moth feed on the leaves when young but older cutworms feed underground and can sever the stems.  A summary of symptoms caused by different caterpillar species on herb crops is given in Table 1.

Table 1. Summary of host crops and damage symptoms caused by caterpillars

Caterpillar type	Host crops	Species	Damage type
Foliar feeder	Many	Silver Y moth Angle-shades moth	Windows and holes in leaf. Frass.
Foliar feeder	Cruciferae only	Small white butterfly Large white butterfly Cabbage moth	Holes in leaf.  Can skeletonise plants. Frass.
Leaf roller /webber	Many	Carnation tortrix moth	Rolls leaves. Ties with silk. Leaf & growing point damage, distortion.
Leaf roller /webber	Mainly mints	Mint moth	Rolls leaves. Ties with silk. Leaf & growing point damage, distortion.
Root/stem feeder	Many	Cutworms (e.g. turnip moth caterpillars)	Minor foliar damage. Sever roots & underground stems, causing plant collapse

  

Sources of infestation and favourable conditions

The sources of infestation are usually caterpillars or pupae that overwinter from the previous season.  Some species e.g. angle-shades moth and carnation tortrix moth overwinter as caterpillars on the host plant.  Others e.g. the small and large white butterfly overwinter as pupae above ground, on cultivated or weed hosts or on man-made structures e.g. fences and buildings.  The exception is the silver Y moth, which does not overwinter in the UK, but migrates in from continental Europe on southerly winds during spring and autumn. 

Adult moths of most species fly into protected herb crops between March and May, depending on species and the season.  The first generation moths usually fly in from outdoors, from fields that grew susceptible crops (or weeds) the previous season, but they can also fly from overwintering sites within the glasshouse or tunnel.  The moths lay their eggs on the foliage, and these hatch into caterpillars that feed on the plants.  When fully fed, the caterpillars pupate on the plants or in the soil, depending on species, and many species produce a second generation of moths and caterpillars between mid-summer and early autumn. Carnation tortrix moth can have up to five generations per year under protection.  Warm conditions favour caterpillar development. 

Cutworm caterpillars, e.g. those of the turnip moth, are rarely a problem on protected herbs.  The adult moths are active from late May to late June and lay their eggs on many common weeds or plants, or on bare soil or plant debris.  The caterpillars feed on the foliage when young, but older cutworms then descend to the soil to feed on stem bases.  Cutworms are only a problem during hot, dry summers with low rainfall during June and July.  Crops watered with overhead irrigation are unlikely to suffer damage as heavy rain or water causes the young caterpillars to drop off the plants and die.

  

Integrated Pest management (protected herbs)

Cultural control

• Maintain strict weed control in and around glasshouses and tunnels, including in empty structures in between crops. 
• Dispose of unwanted infested plants and any plant debris promptly and carefully.
• Pheromone traps are available for some species e.g. cabbage moth, carnation tortrix moth, silver Y moth and turnip moth.  They will trap male moths and are intended as a monitoring system - use for early warning of first moth activity, to help time control measures. The traps themselves are not intended as a control measure; too few moths will be caught to affect the population size.

  

Biological control

1.    Bacillus thuringiensis var. kurstaki (often known as Bt),  (Dipel DF) is a bacterial biopesticide which is specific to caterpillars.  It is applied as a high volume spray and has a SOLA for use on protected and outdoor herbs.  Bt is most effective against young caterpillars. Optimal control relies on good coverage of both sides of the leaves, as the caterpillars need to eat treated foliage to take in the bacteria.  Dipel DF should be sprayed onto dry foliage and plants should not be watered overhead within 6 hours of treatment.  Once ingested Bt quickly stops the caterpillars from feeding, but death can take 1-3 days.  Dead caterpillars shrivel up and can sometimes be seen hanging from the foliage.

2. Entomopathogenic nematodes
Some entomopathogenic nematodes are effective against caterpillars.  HDC project HNS 130 has shown that foliar sprays of Steinernema feltiae (e.g. Nemasys F) will give some control of carnation tortrix moth caterpillars on hardy nursery stock.  Steinernema carpocapsae (‘Capsanem') is now available for the control of caterpillars and is considered to be more effective against caterpillars than S. feltiae.   Product recommendations for Capsanem should be followed carefully for optimum effect.

3. Trichogramma brassicae
is a minute wasp that parasitises moth eggs, supplied as black parasitised eggs on cards. Adult wasps emerge from the eggs and search to find suitable moth eggs that have been laid on the plants. The parasitised eggs turn black and develop into more parasites instead of caterpillars. Trichogramma females lay their eggs inside the eggs of certain moth species, e.g. cabbage moth, silver Y moth and carnation tortrix moth.  Trichogramma will not parasitise all species of moth eggs and experience on crops of herbs is still limited.  HDC project HNS 170 investigated the potential of Trichogramma against tortrix moths on hardy nursery stock.  Details are given in the project report, available from the HDC.

  

Monitoring within IPM

• Check plants regularly (e.g. weekly) for damage symptoms, caterpillars or frass.
• Many caterpillars are nocturnal, so can be difficult to find during the day when they are hiding.
• If using pheromone traps, check regularly for adult moths to help time control measures.

  

Chemical control (protected herbs)

The HDC Pest Bulletin includes a cutworm warning scheme, which gives guidance on timing of control measures if necessary, see:

http://www2.warwick.ac.uk/fac/sci/whri/hdcpestbulletin/

If a pesticide is needed for control of foliar-feeding caterpillars or cutworms, it should be selected carefully, taking into account pesticide compatibility within IPM (see Section A - Principles of IPM) and harvest interval (see Table 2 on the homepage). 

For full, specific information on safety of pesticides to individual biological control agents, consult your biological control supplier.  General guidelines on the selection and compatibility of pesticides within IPM are given below:       

‘Safe' in IPM
There are no chemical pesticides approved for use against caterpillars on protected herbs that are completely safe to biological control agents.  Dimilin Flo (diflubenzuron) has a SOLA for use on outdoor herbs but not protected herbs.

‘Moderately harmful' in IPM

• Tracer (spinosad) has a SOLA for use on protected herbs and is effective against caterpillars.  The product is safe to many biological control agents but is harmful to parasitic wasps e.g. those used for leaf miner or whitefly control, for up to 2 weeks after application.  Although there should be no resistance problems with caterpillars, over-use of Tracer could lead to thrips tolerance of the pesticide.  Thus it is very important to follow the Specific Resistance Management Guidelines given on the SOLA.  Also see Section A - Principles of IPM and Table 2 on the homepage).
• Spruzit (pyrethrins plus oil) is approved for use on any protected edible crop and is effective against caterpillars.  The product is harmful to many biological control agents but has only limited persistence, thus it can be used with care within IPM programmes.  N.B. Spruzit has been known to cause leaf damage to curly parsley.  The product should be tested on a few plants first for any herb species, before using on a large scale, particularly if more than one application is made.  See product label for full details.

‘Harmful' in IPM

• Various pyrethroid products (e.g. cypermethrin and deltamethrin) have SOLAs for use on protected leafy herbs and will control caterpillars. Pyrethroid insecticides are harmful to biological control agents for up to three months after application, thus they are incompatible with IPM.
• Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants. Also see Section A - Principles of IPM.
• Pesticide approval information in this guideline is current at 31 March 2009. 
• Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or with the Information Office at the Pesticides Safety Directorate (PSD) Tel: 01904 640500; or on their website (http://www.pesticides.gov.uk/).
• Always follow label recommendations or statutory conditions for use on Specific Off-Label (SOLA) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or SOLA.
• Growers must hold a paper or electronic copy of the current SOLA before using any product under the SOLA arrangements.  Any use of a pesticide with a SOLA is at grower's own risk.  Relevant SOLAs are sent to HDC members by HDC, or are available from PSD (see above) or from consultants.
• Use pesticides safely.

  

Further information

HDC Factsheet 13/06.  Caterpillar pests of bedding and pot plants.

HDC report on project HNS 170 (2008):  HNS; initial evaluation of Trichogramma brassicae parasitoids for control of carnation tortrix moth.